Also, it's not hard to package all the stuff a language needs and put it into an APK. It's just messy and there are a lot of "edge cases" and things to work out (like the problem of interacting with the Platform, you'd need to call into a "Java" interface from your language, you'd need to run your app inside ART/Dalvik, maybe fork - https://stackoverflow.com/questions/39006044/repeatedly-fork... ).
And so if you spend half a year just getting something to run, and on the next Android version Google makes a change, and bamm, you might get left in the cold, because security or whatever.
Replicant is the most "open".
Also, it's not hard to package all the stuff a language needs and put it into an APK. It's just messy and there are a lot of "edge cases" and things to work out (like the problem of interacting with the Platform, you'd need to call into a "Java" interface from your language, you'd need to run your app inside ART/Dalvik, maybe fork - https://stackoverflow.com/questions/39006044/repeatedly-fork... ).
And so if you spend half a year just getting something to run, and on the next Android version Google makes a change, and bamm, you might get left in the cold, because security or whatever.